1. Field of the Invention
The present invention relates to an electric power steering apparatus using an electric motor as a source for generating a steering assist force.
2. Description of Related Art
An electric power steering apparatus for vehicles is, for example, constructed so as to detect steering torque applied to an input shaft, based on a relative angular displacement between the input shaft connected to a steering wheel and an output shaft connected coaxially to the input shaft through a torsion bar, to drive a steering force assisting electric motor based on the detected torque, and to transmit the torque of this motor to a steering mechanism through a reduction gear mechanism. In such a structure, the operation of the steering mechanism corresponding to the rotation of the steering wheel is assisted by the rotation of the electric motor, and the labor of a driver for steering is reduced.
The reduction gear mechanism comprises a worm coupled to the driving shaft of the steering assisting electric motor, and a worm wheel meshed with this worm. This worm wheel is fitted and fixed at a midway point of a transmission path running from the output shaft to the steering mechanism.
Further, both ends of the worm are supported by a pair of rolling bearings to improve the rotating performance of the worm. However, the rolling bearing has a thrust clearance between an inner ring and outer ring fitted with a plurality of rolling elements therebetween, and it is necessary to eliminate the thrust clearance for the following reasons. Specifically, it is necessary to eliminate the thrust clearance because the thrust clearance of the rolling bearing causes a backlash of the worm in the axial direction, and abnormal noise caused by the backlash leaks into the room of a vehicle and makes the driver uncomfortable.
For such reasons, there is provided a screw ring which is in contact with one end of the outer ring of one rolling bearing supporting one end of the worm and capable of being turned. By turning this screw ring, the outer ring and inner ring of the one rolling bearing are moved relatively in the axial direction, and also the inner ring and outer ring of the other rolling bearing are moved relatively in the axial direction through the worm, thereby eliminating the thrust clearance and backlash of the worm in the axial direction.
By the way, the worm of the electric power steering apparatus constructed as described above is supported so that the worm can not move in the axial direction relative to the rolling bearings that support both ends thereof. Therefore, in the case where the electric power steering apparatus is constructed so that the electric motor is rotated from an early stage of steering and steering is assisted by turning the steering wheel in the left or right direction from the steering neutral position, the steering assist is provided even when the steering wheel is turned at a small angle of around 1xc2x0 during driving of the vehicle at high speed, causing a reduction in the steering feeling. For this reason, in general, the electric power steering apparatus is constructed so that the electric motor is not driven when the steering angle is as small as around but is driven when the steering angle exceeds a suitable steering angle.
In such a structure where the electric motor is not driven until the steering angle exceeds a suitable angle, however, in a steering range in which the electric motor is not driven, i.e., steering in a range near the steering neutral position, the steering force of the steering wheel is transmitted to the driving shaft of the electric motor through the input shaft, torsion bar, output shaft, worm wheel and worm sequentially, and consequently the driving shaft is rotated. As a result, load to rotate the driving shaft of the electric motor is applied to the steering wheel through the worm, worm wheel, output shaft, torsion bar and input shaft sequentially, and the steering load increases.
By the way, in order to decrease the steering load in the steering range in which the electric motor is not driven, for example, there is a known technique disclosed in Japanese Patent Application Laid-Open No. 11-43062 (1999). More specifically, this technique adopts a structure where two rolling bearings separated in the axial direction support the worm coupled to the driving shaft of the electric motor so that the worm is movable in the axial direction, two plate springs are provided between the worm and the inner rings of the two rolling bearings, and the inner rings are forced in the axial direction relative to the outer rings by the elastic restoring force of the plate springs. Accordingly, it is possible to eliminate the thrust clearances of the rolling bearings and restrain the movement of the worm in both directions along the axial direction, thereby solving the above problems.
In this structure, when the steering force of the steering wheel is transmitted from the worm wheel to the worm by steering in a steering range in which the electric motor is not driven, the worm overcomes the elastic restoring force of the plate springs by a component force applied to the worm in the axial direction and moves in the axial direction, the rotation angle of the worm becomes smaller, and the transmission power from the worm to the driving shaft of the electric motor is reduced.
However, in the structure as disclosed in the Japanese Patent Application Laid-Open No. 11-43062 (1999), two rolling bearings and two plate springs are necessary to eliminate the thrust clearances of the rolling bearings, resulting in a complicated structure.
Moreover, since the plate springs for restraining the movement of the worm in both directions along the axial direction also perform the function of eliminating the thrust clearances of the rolling bearings, the elastic restoring force of the plate springs needs to be set relatively high, and accordingly the effect of reducing the steering load in the steering range in which the electric motor is not driven is reduced.
The present invention has been made with the aim of solving the above problems, and it is an object of the present invention to provide an electric power steering apparatus capable of decreasing a steering load in a steering range in which a steering assisting electric motor is not driven, even when the electric power steering apparatus comprises a single rolling bearing for supporting a worm which is rotated by the steering assisting electric motor.
An electric power steering apparatus according to the first aspect of the present invention comprising a worm that is rotated by a steering assisting electric motor, bearing means for supporting the worm, and a worm wheel which meshes with the worm and is connected to a steering mechanism, for assisting steering by the rotation of the electric motor, characterized in that the bearing means comprises: a rolling bearing with an inner ring into which a shaft portion on one end side of the worm is inserted, for supporting the worm rotatably; a first resilient member disposed in contact with an inner circumference of the inner ring of the rolling bearing, and capable being extended in a axial direction of the worm by a force applied to the worm in the axial direction; and two second resilient members for restraining a movement of the worm in the axial direction, the second resilient members being held on the worm or the inner ring so as to be in contact with both ends of the inner ring of the rolling bearing.
In the electric power steering apparatus of the first aspect of the present invention, the first resilient member is in contact with the inner circumference of the inner ring of one rolling bearing, and the second resilient members are in contact with both ends of the inner ring of the rolling bearing. Therefore, with a force in the axial direction applied to the worm when steered in either the left or right direction from a steering neutral position, the first resilient member is extended, and either of the second resilient members is also bent. As a result, even when the bearing means for supporting the worm comprises a single rolling bearing, it is possible to decrease the steering load in a steering range in which the electric motor is not driven. Moreover, since the load generated by the worm is shared by the first resilient member and the second resilient members, it is possible to improve the durability of the first resilient member and the second resilient members, and the electric power steering apparatus can be used over a long period of time.
An electric power steering apparatus according to the second aspect of the present invention is based on the first aspect, and characterized in that the first resilient member and the second resilient members are formed integrally as a movement-restraining member.
According to the electric power steering apparatus of the second aspect of the present invention, in the first aspect, the positions of the first resilient member and the second resilient members can be maintained by holding the first resilient member on the worm, and it is thus possible to omit a process for holding the first resilient member and second resilient members. Further, it is possible to assemble the first resilient member and two second resilient members by an operation for holding the first resilient member on the worm, i.e., by a single operation. Therefore, although the electric power steering apparatus comprises the first resilient member and two second resilient members, it is possible to improve the efficiency of the assembling operation and reduce the cost of the electric power steering apparatus.
An electric power steering apparatus according to the third aspect of the present invention is based on the first or second aspect, and characterized in that one of the second resilient members has a smaller movement restraining force in the axial direction of the worm than the other.
According to the electric power steering apparatus of the third aspect of the present invention, in the first or second aspect, with a force in the axial direction applied to the worm when steered in either the left or right direction from the steering neutral position, the first resilient member is extended and one of the second resilient members is bent. Moreover, since one of the second resilient members has a smaller movement restraining force in the axial direction of the worm, it can be easily mounted on the worm, and the steering load in a steering range in which the electric motor is not driven can be decreased. Furthermore, by forming the first resilient member and second resilient members integrally, the second resilient member prevents the first resilient members from coming out of the inner ring, thereby making it possible to maintain the first resilient member and second resilient members in appropriate positions over a long period of time.
Besides, in the third aspect, when both end portions of the worm are supported by rolling bearings and the first resilient member and second resilient members are provided between the worm and the inner rings of the two rolling bearings, it is possible to equalize the forces for restraining the movement of the worm with respect to a certain steering angle in the left and right directions from the steering neutral position, thereby improving the steering feeling.
Further, in this case, since the first resilient member can be held by being fitted on the worm, it is possible to eliminate a process for holding the first resilient member. In addition, the second resilient members can be assembled with the operation for holding the first resilient member, i.e., the first resilient member and second resilient members can be assembled simultaneously by a single operation. Therefore, although this structure comprises the first resilient member and two second resilient members, it is possible to improve the efficiency of the assembling operation and reduce the cost of the electric power steering apparatus.
An electric power steering apparatus according to the fourth aspect of the present invention is based on the first or second third aspect, and characterized in that the first resilient member comprises: a cylindrical portion; and an annular projecting portion projecting in a radial direction of the worm, on an outer circumference of the cylindrical portion.
According to the electric power steering apparatus of the fourth aspect of the present invention, in the first or second aspect, with the application of a force in the axial direction to the worm, the cylindrical portion is extended and the annular projecting portion on the outer circumference of the cylindrical portion is also extended. It is therefore possible to gradually decrease the steering load in a steering range in which the electric motor is not driven and further improve the steering feeling.
An electric power steering apparatus according to the fifth aspect of the present invention is based on the first or second aspect, and characterized by comprising a ring member between an inner circumference of the first resilient member and the worm.
According to the electric power steering apparatus of the fifth aspect of the present invention, in the first or second aspect, the ring member can be fitted and held on the worm without bringing the first resilient member into contact with the worm, and therefore the first resilient member and second resilient members can be assembling more easily.
An electric power steering apparatus according to the sixth aspect of the present invention is based on the first aspect, and characterized in that the first resilient member and the second resilient members are O rings.
According to the electric power steering apparatus of the sixth aspect of the present invention, in the first aspect, ready made O rings can be used as the first resilient member and the second resilient members, and therefore it is possible to reduce the costs of the first resilient member and second resilient members and consequently the cost of the electric power steering apparatus.
An electric power steering apparatus according to the seventh aspect of the present invention is based on the first aspect, and characterized in that the bearing means comprises: a rolling bearing with an inner ring into which a shaft portion on one end side of the worm is inserted for supporting the worm rotatably, and for supporting the worm so that the worm is movable in the axial direction; forcing means for forcing the inner ring of the rolling bearing in the axial direction of the worm; and resilient members for restraining a movement of the worm in both directions along the axial direction in a state in which a movement of the inner ring of the rolling bearing due to the forcing means is stopped.
According to the electric power steering apparatus of the seventh aspect of the present invention, in the first aspect, the forcing means forces the inner ring of the rolling bearing in the axial direction, and therefore it is possible to eliminate the thrust clearance of the rolling bearing. Moreover, since the resilient members for restraining the movement of the worm in both directions along the axial direction restrain the movement of the worm in the axial direction in a state in which the movement of the inner ring of the rolling bearing due to the forcing means is stopped, the thrust clearance of the rolling bearing supporting the worm can be eliminated, and further the elastic restoring force of the resilient member can be set irrespective of the forcing means. It is therefore possible to decrease the steering load in a steering range in which the electric motor is not driven, equalize the bent amounts of the resilient members with respect to a certain steering angle in the left and right directions from the steering neutral position, and improve the steering feeling.
An electric power steering apparatus according to the eighth aspect of the present invention is based on the seventh aspect, and characterized in that the bearing means comprises a sliding bearing into which the other end portion of the worm is inserted.
According to the electric power steering apparatus of the eighth aspect of the present invention, in the seventh aspect, although it is possible to eliminate the thrust clearance of the rolling bearing and decrease the steering load in a steering range in which the electric motor is not driven, the cost of the bearing means and consequently the cost of the electric power steering apparatus can be reduced compared to the structure using two rolling bearings, and further the size and weight of the worm supporting portion can be decreased.
An electric power steering apparatus according to the ninth aspect of the present invention is based on the seventh or eighth aspect, and characterized in that the resilient members are positioned in contact with a ring member provided on the bearing means and the worm, respectively.
According to the electric power steering apparatus of the ninth aspect of the present invention, in the seventh or eighth aspect, the resilient members can be positioned without being affected by the position of the rolling bearing, and therefore it is possible to increase the flexibility in design and easily meet the demand of the users.
An electric power steering apparatus according to the tenth aspect of the present invention is based on the ninth aspect, and characterized in that the ring member is the inner ring.
According to the electric power steering apparatus of the tenth aspect of the present invention, in the ninth aspect, the positions of the resilient members can be maintained by the inner ring of one rolling bearing that supports one end portion of the worm, and therefore it is possible to further simplify the structure and further reduce the cost.
An electric power steering apparatus according to the eleventh aspect of the present invention is based on the eighth aspect, and characterized in that the resilient members are held on the worm, the sliding bearing, or a ring member so that the resilient members are positioned on both end portions of the sliding bearing and/or both end portions of the ring member which supports the sliding bearing.
According to the electric power steering apparatus of the eleventh aspect of the present invention, in the eighth aspect, the positions of the resilient members can be maintained by one sliding bearing that supports the other end side of the worm, and therefore it is possible to further simplify the structure and further reduce the cost.
An electric power steering apparatus according to the twelfth aspect of the present invention is based on the first or seventh aspect, and characterized in that said worm is capable of being tilted about a portion supported by said rolling bearing, and pushing means for pushing said worm toward said worm wheel is further provided.
According to the electric power steering apparatus of the twelfth aspect of the present invention, in the first or seventh aspect, by pushing the worm toward the worm wheel by the pushing means, it is possible to adjust the distance between the centers of rotation of the worm and worm wheel, and decrease the amount of backlash in the meshing portion of the worm and worm wheel.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.